Go to Options >Security Options >General Settings. Next to password Select disable. Then enter your password. If it won't allow you to do this is it possible an old BES policy is preventing it? Have you had the BB since new? On that same password setting page, all the way at the bottom, does it mention anything about an IT Policy?

((Edit)) if you've received a refurbished unit as a warranty replacement, some have reported that password requirements are still in place from old policies even though the policy appears to have been removed. I've never seen it myself but some have posted that.